This invention relates to a system for switching a speech channel between loosely coupled radio base stations, and more particularly to a system for switching a speech channel that, even when a soft hand off instruction signal from a radio base station, which currently supplies speech communication service, does not reach the mobile telephone terminal, enables soft hand off control without causing the cut off of signal transmitted between the mobile telephone terminal and the radio base station.
Soft hand off conforming to IS (Interim Standard)-95 specifications of TIA (TELECOMMUNICATIONS INDUSTRY ASSOCIATION)/EIA (ELECTRONIC INDUSTRY ASSOCIATION) is known in the art.
FIG. 1 is an illustration showing a conventional mobile telephone communication system to which soft hand off conforming to TIA/EIA IS-95 specifications is applied. The mobile telephone communication system conforming to TIA/EIA IS-95 specifications is a mobile telephone communication system of CDMA (code division multiple access) system which has a plurality of service areas 30a to 90a and a radio base station control unit 8. Radio base stations 3a, 4a, 5a, 6a, 7a, 8a, and 9a are provided on the service areas 30a, 40a, 50a, 60a, 70a, 80a, 90a, respectively. In fact, a plurality of radio base station control units 8 are installed and are connected to a plurality of radio base stations 3a and 9a to provide a wider range of service areas.
Different pilot channels are allocated to respective neighbor radio base stations. Specifically, pilot channels 3ap, 4ap, 5ap, 6ap, 7ap, 8ap, and 9ap are allocated respectively to radio base stations 3a, 4a, 5a, 6a, 7a, 8a, and 9a to perform transmission, and a mobile telephone terminal 7 exists which moves between the service areas.
FIG. 2 shows an example of conventional hand off table. The radio base station control unit 8 previously catalogs in the hand off table, as candidate stations for soft hand off, neighbor radio base stations, to which hand off is possible, among radio base stations to be controlled by the radio base station control unit 8.
The radio base station control unit 8 has a threshold for the addition of a speech channel and a threshold for the deletion of a speech channel used at the time of the switching of the speech channel. In practicing the soft hand off, the radio base station control unit 8 sends, as a hand off instruction signal, the threshold for the addition of a speech channel and the threshold for the deletion of a speech channel through a plurality of radio base stations to the mobile telephone terminal 7. The mobile telephone terminal 7 measures the receive field strength of each of a group of pilot channels designated by the radio base station control unit 8. When the measurement results are such that there is a pilot channel having a field strength above the threshold for the addition of a speech channel or a pilot channel having a field strength below the threshold for the deletion of a speech channel, the mobile telephone terminal 7 informs the radio base station control unit 8 of the field strength level of the pilot channel, requesting the switching of the speech channel.
Each radio base station sends information on pilot channel, system information, and field strength measurement list necessary for specifying a radio base station used at the time of initiation of call processing over paging channel (P-ch) to the mobile telephone terminal which exists in the service area of the radio base station. The mobile telephone terminal 7 present within the service area 80a recognizes from a signal on P-ch that the radio base station used at the time of the initiation of call processing is a radio base station 8a and that the field strength should be measured for pilot channels 3ap, 7ap, 8ap, and 9ap. 
As soon as the request for call connection from the mobile telephone terminal 7 is sent to the radio base station control unit 8 through the radio base station 8a, the radio base station control unit 8, upon the completion of the establishment of the speech channel in the radio base station 8a, sends a neighbor list signal to the mobile telephone terminal 7. The mobile telephone terminal 7 again obtains the field strength measurement list, and measures the receive field strength of each of a group of pilot channels designated by the radio base station control unit 8. When the mobile telephone terminal 7 is moved from the radio base station 8a towards the radio base station 3a, due to the approach of the mobile telephone terminal 7 to the radio base station 3a, the pilot receive field strength of the pilot channel 3ap received by the mobile telephone terminal 7 exceeds the threshold for the addition of a speech channel. The mobile telephone terminal 7 sends a pilot field strength level report to the radio base station 8a. The radio base station 8a sends, to the radio base station control unit 8, the pilot field strength level report received through an assembled line 9 containing control signal lines and voice signal lines. The radio base station control unit 8 specifies from the received pilot field strength level report and the hand off table that, for the radio base station 8a, candidate radio base stations for hand off are 3a, 7a, and 9a and that, among the specified candidate radio base stations for hand off, the pilot channel having a pilot receive field strength above the threshold for the addition of a speech channel is the pilot channel 3ap. The radio base station control unit 8 adds the speech channel of the radio base station 3a, and advises the mobile telephone terminal 7 through a hand off instruction signal that the speech channel of the radio base station 3a and the speech channel of the radio base station 8a would be used in soft hand off after that. After the advice, the radio base station control unit 8 also sends a neighbor list signal. Thus, the mobile telephone terminal 7 obtains a field strength measurement list after the switching of the speech channel.
The mobile telephone terminal 7 measures the receive field strength of each of a group of pilots designated by the radio base station control unit 8. As the mobile telephone terminal 7 is further moved towards the radio base station 3a, the pilot receive field strength of the pilot channel 8ap becomes a value below the threshold for the deletion of the speech channel. In this case, the mobile telephone terminal 7 sends a pilot field strength level report to the radio base station 3a, and the radio base station 3a sends the pilot field strength level report through the assembled line 9 to the radio base station control unit 8. The radio base station control unit 8 judges that, from the hand off table, for the radio base station 3a, the radio base stations 4a, 5a, 6a, 7a, 8a, and 9a are candidate base stations for hand off, that the addition of a speech channel is not carried out because the pilot receive field strength of the specified candidate radio base stations for hand off does not exceed the threshold for the addition of the speech channel, and that, from the received pilot field strength level report, the speech channel of the radio base station 8a having a field strength below the threshold for the deletion of the speech channel should be deleted. The radio base station control unit 8 advises the mobile telephone terminal 7 through a hand off instruction signal that only the speech channel of the radio base station 3a is usable by the mobile telephone terminal 7. Thereafter, the radio base station control unit 8 deletes the speech channel of the radio base station 8a, and then sends a neighbor list signal to the mobile telephone terminal 7. Thus, the mobile telephone terminal 7 obtains a field strength measurement list to be applied after the switching of the speech channel.
The radio base station control unit 8 executes soft hand off control through the addition and deletion of speech channels in the manner described above.
In the CDMA type mobile telephone communication system, when there is a tunnel, a radio base station, of which the service area is an area within the tunnel, is installed. In this case, however, the receive field strength of the pilot channel is abruptly varied at the entrance and exist of the tunnel.
The mobile telephone terminal measures the pilot receive field strength of a pilot channel sent from a radio base station which currently supplies speech communication service. When the pilot receive field strength has become a value below the threshold for the deletion of a speech channel, the mobile telephone terminal starts up a hand off drop timer in order to judge whether or not a pilot field strength level report should be sent through the radio base station, which currently supplies speech communication service, to the radio base station control unit. Until the hand off drop timer times out, the field strength is continuously measured to examine whether or not the pilot receive field strength of the pilot channel, which has been once lowered to a value below the threshold for the deletion of a speech channel again exceeds the threshold for the deletion of a speech channel. Upon the time-out of the hand off drop timer, the mobile telephone terminal sends the pilot field strength level report to the radio base station control unit, and is then transferred to the status of a wait for an instruction of soft hand off from the radio base station control unit.
However, an abrupt variation in the pilot receive field strength of the pilot channel of the radio base station which currently supplies speech communication service, upon the time-out of the hand off drop timer, makes it impossible for the mobile telephone terminal to communicate with the radio base station which has supplied speech communication service up to now. As a result, the pilot field strength level report sent from the mobile telephone terminal does not reach the radio base station which currently supplies speech communication service, or alternatively, the soft hand off instruction signal from the radio base station, which currently supplies speech communication service, does not reach the mobile telephone terminal 7. Therefore, the soft hand off control cannot be carried out, unfavorably leading to call disconnection.
The occurrence of this problem is not limited to the inside of the tunnel. For example, when a mobile telephone terminal currently in speech communication through a radio base station installed on a hill descends from a considerably steep slope, the pilot receive field strength of the radio base station, which currently supplies speech communication service, is abruptly lowered and, in addition, the pilot receive field strength of the pilot channel of a radio base station installed on the bottom of the slope suddenly exceeds the threshold for the addition of a speech channel. In this case, as with the phenomenon caused within the tunnel, upon the time-out of the hand off drop timer, the mobile telephone terminal becomes uncommunicable with the radio base station which has supplied speech communication service up to now.
Accordingly, it is an object of this invention to provide a mobile telephone communication system which, even when an abrupt variation in the pilot receive field strength of a radio base station in its pilot channel, which currently supplies speech communication service to a mobile telephone terminal during movement, has made it possible for a pilot field strength level report sent from the mobile telephone terminal to reach the radio base station, which currently supplies speech communication service, or a soft hand off instruction signal from the radio base station, which currently supplies speech communication service, to reach the mobile telephone terminal, enables soft hand off control without causing cut off of signal transmission between the mobile telephone terminal and the radio base station.
According to the invention, a system for switching a speech channel between loosely coupled radio base stations, comprises:
a radio base station control unit which allows a mobile telephone terminal to have a communication channel through a plurality of radio base station devices; and
a radio base station which is provided with a field strength measurement-only channel which allows the mobile telephone terminal to measure the field strength of radiowave to reach the mobile telephone terminal;
wherein the mobile telephone terminal measures the field strength in field strength measurement-only channel designated by the radio base station control unit and informs the radio base station control unit of the measurement results through the radio base station, and
the radio base station unit is provided with a hand off table for recording a soft hand off candidate station for the each radio base station, and an additional station search table for recording an additional radio base station that the candidate station is expected to next perform soft hand off, and, regardless of the field strength, allows the mobile telephone terminal to have a speech channel to the additional station.